Current hypotheses suggest that both the plasmin system and metalloproteinases are involved in tumor invasion of basement membranes. Plasmin has been reported to degrade BM glycoproteins and activate collagenolytic metalloproteinases. Alternatively, it has been suggested that it is not necessary to postulate involvement of specific type IV collagenases in tumor cell-mediated degradation of type IV collagen. In this study, we demonstrated that plasmin can directly degrade native and denatured type IV collagen in solution and in tissue sections. Tumor cell lines secreted plasminogen activators into culture supernatants that activated exogenous plasminogen to degrade type IV collagen in zymograms and to remove type IV collagen immunoreactivity from tissue sections. Inhibition of metalloproteinase activity in culture supernatants by the addition of EDTA did not interfere with plasminogen mediated type IV collagen degradation. Thus, we propose that a metalloproteinase independent route for type IV collagen degradation exists, dependent on plasminogen conversion to plasmin. We are currently evaluating whether this metalloproteinase independent collagenolytic activity facilitates tumor invasion of extracellular matrices in vitro.